This invention relates to tip treatment bars of a rotor casing for a gas turbine engine.
WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor through a series of slots defined between solid tip treatment bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
In a gas turbine engine, blades of a rotating stage may become damaged or become detached from the rotating hub on which they are mounted. Damage of this type may be caused, for example, by impact or foreign object damage such as a bird strike. The blade, or fragment of a blade, which is shed, can cause catastrophic damage to other parts-of the engine, or even break through the engine casing and puncture the aircraft skin. The consequential effects of blade shedding can be particularly serious if the blade in question is in the compressor stage of the engine, and particularly near the front of the compressor stage, since such blades are the largest and heaviest in the engine.
Known tip treatment bars are solid and relatively robust and, in general, are as able as the adjacent parts of the casing to withstand impact from detached blades or blade fragments.
Another problem encountered with known tip treatments is high cycle fatigue failure. As the ends of the blades pass the tip treatment bars, the bars are aerodynamically excited. The resulting vibrations cause fatigue cracking in the bars, particularly at their ends.
It is an object of the present invention to reduce the strength of tip treatment bars.
A further object of the present invention is to increase the resistance of tip treatment bars to high cycle fatigue stress.
A more specific object of the present invention is to provide tip treatment bars which are sufficiently fragile so as to fracture under the impact of fragments of turbine blades, yet are resistant to fatigue damage.
According to the present invention there is provided a tip treatment bar for use in a gas turbine engine casing. The bar has a cavity which is open on one side of the bar. The cavity is filled with a damping material.
The cavity may extend lengthwise of the bar and may be divided by one or more partitions into separate cells spaced lengthwise of the bar.
The channel may be either open or closed at the ends of the bar.
The bar may be of rectangular cross-section, with the cavity opening at a shorter side of the cross-section.
The damping material reduces vibration amplitudes in the bar caused by aerodynamic excitation, thus reducing high cycle fatigue failure of the tip treatment bars.
The cavities in the tip treatment bars weaken them so that their resistance to impact from detached blades or blade fragments is substantially less than similar solid bars. Preferably, the damping material does not contribute significantly to the impact strength of the tip treatment bar under blade or blade tip release conditions. Consequently, such blades or blade fragments may break through the tip treatment bars, with minimal loss of kinetic energy, to be brought to rest by an external containment system such as a Kevlar wrap.
The bars may be formed from metal or from a composite material. Preferably, the damping material is a visco elastic material with a high temperature capability. A suitable material is a silicone moldmaking rubber, for example the material available from the Dow Corning Corporation under the name SILASTIC J(copyright).
The tip treatment bars may be mounted between end supports connected to, or forming part of, the casing of the engine. The tip treatment bars may be integral with the end supports. Alternatively, the bars may be formed separately and subsequently assembled with the end supports.